1. Field of the Invention
The present invention relates to a method for forming a polycide structure. More particularly, the present invention relates to a method for forming a polycide structure including tungsten silicide (WSix) in a semiconductor device.
2. Description of the Related Art
As information media, such as computers, become more widely used, a semiconductor device is required that operates at a high speed, a low voltage, and has a large storage capacitance. Accordingly, both the size of a pattern formed on a semiconductor substrate, which is fabricated as the semiconductor device, and the spacing between such patterns have become smaller.
In conventional semiconductor device fabrication, polysilicon is widely used for forming electrical wiring members, such as a gate electrode or a bit line. In order to reduce the size of and spacing between patterns, however, a lower resistant material is required for the electrical wiring members.
Recently, a polycide material has been used for forming the electrical wiring members. The polycide material has electrical features substantially identical to those of the polysilicon material except that a resistivity of the polycide material is significantly lower than a resistivity of the polysilicon material. Typically, the polycide material is deposited as a composite layer consisting of a polysilicon material doped with impurities and a refractory metal silicide material having a high melting point. The composite layer may be used as a gate electrode or a bit line of a highly integrated circuit. Preferably, the refractory metal silicide material is selected from tungsten (W), molybdenum (Mo), titanium (Ti) and tantalum (Ta).
In order to form the gate electrode or the bit line using the refractory metal silicide material, a lower pressure chemical vapor deposition process or a plasma enhanced chemical vapor deposition process is used. Among the refractory metal silicide materials, the tungsten silicide material has superior characteristics when used in a combined form with the polysilicon material. That is, when tungsten silicide is bonded to polysilicon, the resulting material has superior characteristics for self-passivation, stability against wet chemicals, surface roughness, adhesion, anti-oxidation and reproducibility. Typically, the tungsten silicide (WSix) material is obtained by reacting silane (SiH4) gas with tungsten hexafluoride (WF6) gas and is deposited on a semiconductor substrate by means of the chemical vapor deposition process.
FIGS. 1A to 1C illustrate sectional views showing steps of a conventional method for forming a polycide structure including tungsten silicide.
Referring to FIG. 1A, a polysilicon film 12 is formed on a semiconductor substrate 10. Polysilicon film 12 is obtained by depositing polycrystalline silicon doped with high-density impurities on the semiconductor substrate 10.
Referring to FIG. 1B, a tungsten silicide precursor layer 14 is formed on an upper surface of the polysilicon film 12 using the aforementioned silane gas and tungsten hexafluoride (WF6) gas deposition process. The silane gas is supplied first in order to create a deposition atmosphere at the upper surface of polysilicon film 12. The silane gas is supplied at a flow rate of 350 to 450 sccm in a chamber having a pressure of 0.7 to 1.0 Torr and a temperature of 350 to 450xc2x0 C. for 5 to 10 seconds. The WF6 gas is then added at a flow rate of 3 to 4 sccm while maintaining the chamber pressure and temperature conditions described above.
Referring to FIG. 1C, a polycide pattern 18 is formed by sequentially etching the tungsten silicide precursor layer 14 and a predetermined portion of the polysilicon film 12 using a hard mask 16 made of an insulating film as an etching mask. In a final step, an oxide film 20 is formed by heat-treating the polycide pattern 18 in an oxygen atmosphere without removing hard mask 16.
In this step, oxygen gas is supplied into the chamber while the chamber temperature is maintained at 1000 to 1500xc2x0 C. This heat-treating causes silicon materials contained in a patterned tungsten silicide precursor layer 14 and a patterned polysilicon film 12 to be reacted with the oxygen gas such that an oxide film 20 is formed at sidewalls of both the patterned tungsten silicide precursor layer 14 and the patterned polysilicon film 12, as well as on an upper surface of the semiconductor substrate 10. Thus, a conventional polycide structure 22 is formed.
However, when the heat-treating process is carried out under the oxygen atmosphere, if the silicon material is over-reacted to the oxygen gas, silicon contained in the patterned tungsten silicide precursor layer 14 is oxidized such that the amount of silicon contained in the patterned tungsten silicide precursor layer 14 is insufficient for being reacted with tungsten. If the amount of silicon in the patterned tungsten silicide precursor layer 14 is insufficient for reaction with tungsten, then silicon moves to the patterned tungsten silicide precursor layer 14 through an interfacial surface. This movement of silicon generates a vacancy in the patterned polysilicon film 12. In extreme conditions, a void 24 is created in the patterned polysilicon film 12, thereby causing a failure of the semiconductor device. The failure becomes more severe as the heat-treating time and the heat-treating temperature are increased and as the design rule of the polycide structure 22 is decreased.
In order to solve the foregoing problems, it is a feature of an embodiment of the present invention to provide a polycide structure made of tungsten silicide and capable of preventing void formation during a heat-treating process.
An embodiment of the present invention provides a method for creating a polycide structure including: 1) sequentially depositing: a polysilicon film doped with impurities, a seed film containing a sufficient amount of amorphous silicon for being reacted with tungsten, and a tungsten silicide precursor layer on a semiconductor substrate; and 2) converting the tungsten silicide precursor layer to tungsten silicide. Since the seed film has a sufficient amount of amorphous silicon for being reacted with tungsten, the void formation in the polysilicon film during the converting process is prevented.
The deposition of the seed film and the tungsten silicide precursor layer are preferably conducted in a common chamber at a same temperature.
These and other features of the present invention will be readily apparent to those of ordinary skill in the art upon review of the detailed description that follows.